The so-called SOS substrate is used in a high frequency switching IC or the like. The SOS substrate is an adhered substrate that includes a base substrate of sapphire having characteristics of high insulation, low dielectric loss and high thermal conduction, and a silicon thin film formed on the surface of the base substrate to configure a semiconductor device. Conventionally, a main method for forming the SOS substrate involves forming a silicon layer on the base substrate by epitaxial growth. On the other hand, in recent years, a method that bonds the silicon layer directly onto the base substrate has been developed to contribute to improve the performance of semiconductor devices (Patent Documents 1, 2, and 3).
With the progress in bonding techniques, various base substrates other than that made of sapphire have been proposed. Among them, polycrystalline transparent alumina has been used for a luminous vessel for a high-luminance discharge lamp and a dummy wafer of a semiconductor manufacturing device. The polycrystalline transparent alumina is produced by densely sintering a high-purity raw material under a reducing atmosphere at a high temperature. The thus-produced polycrystalline transparent alumina has the advantages of eliminating the necessity for a high-cost crystal growing step while maintaining the excellent characteristics of high insulation, low dielectric loss and high thermal conduction comparable with those of the sapphire (Patent documents 4, 5 and 6).
In fabricating the adhered substrate by directly bonding the silicon layer to the base substrate, a difference in thermal expansion between the base substrate and the silicon layer or the like might cause poor adhesion at the outer periphery of the adhered substrate. The silicon layer may thereby be peeled off from the base substrate in a mechanical process or etching process after bonding (Patent documents 7 and 8).